This thread is certainly interesting. Some who always use it, some who never use it and some who use it sometimes.
As mentioned above, I am in the never camp.
There are some people in this thread that I have admiration of their opinion, but I really am beginning to wonder it this is a case of "I've always done it, so it must be right" with those that have used them for years.
I say that because I went out and purposely directed multiple optics in the sun, from multiple angles on multiple days, at sun-up, sun-down and everything in between and I never saw a sunshade or ARD do hardly anything at all. Perhaps if the angle was correct to within 1 degree (yeah, I am spit-balling that measurement) it could make the slightest difference. It's like illumination to me, if the scope I have has good glass and is bright, there is about 1 minute in the morning and 1 minute in the evening where illumination is NOT detrimental. (I am not taking into considering the need for illumination on FFP scopes for something like LPVO use)
The scopes that had higher-end glass, simply performed better when facing all angles of the sun, and I do mean better than the lower end scopes WITH sunshades.
As to saying there is no downside, I disagree, the one difference a long sunshade made was reducing light to my eyeball as the sun went down.
With that being said, maybe I need to revisit and revise my testing, but if anything, the best thing I got out of this was the neoprene koozie. Seems like a simple value add that I could carry in a pack just in case I want to give it a shot when encountering some annoying flare/ghosting.